Solar power has become an increasingly important energy source for the world. But like many other forms of energy, electricity produced by photovoltaic power systems is a scarce and valuable resource. Although solar power is renewable and pollution free, fixed costs associated with generating solar power are high. To provide more of a scarce resource and to offset these high fixed costs, a solar power system should operate to maximize its power output when possible.
Photovoltaic power systems generate power by converting solar energy into electricity. Solar panels containing photovoltaic cells are typically arranged in an array and constructed at a location that receives plentiful sunshine. Photons from the sun create a voltage in the photovoltaic cells, which produce a direct current when connected to a load. Oftentimes, the direct current is converted into an alternating current so that the solar array may provide electricity to a power grid.
A solar array generates maximum power when its photovoltaic cells operate where dI/dV=−I/V, which occurs when the instantaneous slope of the array's power-voltage curve is equal to zero. This maximum power point may vary with solar irradiance and other factors, such as ambient temperature. Maximum power point tracking (MPPT) methods attempt to determine this ideal operating point and adjust how the solar array operates so that the photovoltaic cells take full advantage of available solar energy.
The most widely adopted MPPT methods track a solar array's power-maximizing point reasonably well when solar irradiance and ambient temperature do not vary quickly with time. However, these methods have considerable drawbacks, including relatively poor performance under dynamic conditions. One existing MPPT method is the perturb and observe method, in which the operating voltage or current of an array is adjusted and the power output is observed to determine whether the change results in more power. Although the perturb and observe method may operate the solar array near its maximum power point when irradiance is constant, the solar array's operating power generally oscillates around the maximum power point as the solar array's operating voltage or current is periodically perturbed to determine whether another point maximizes power output. Additionally, during rapidly varying irradiance levels, this method may react too slowly to successfully determine the maximum power point and may even track in the wrong direction.
Another existing MPPT method is the incremental conductance method in which a solar array's power-voltage curve is observed and a maximum power point is found by comparing the solar array's instantaneous conductance (I/V) with an incremental conductance (dI/dV). If the solar array experiences a change in current, its operating voltage is adjusted until dI/dV=−I/V once again. The incremental conductance method improves upon the perturb and observe method in that it does not oscillate around the maximum power point during steady-state operation. However, measuring incremental conductance takes a finite amount of time, during which changes in irradiance may cause the solar array to operate below its maximum power point. As with the perturb and observe method and other MPPT methods, the incremental conductance method does not optimize a solar array's power output when it is unable to accurately track the solar array's maximum power point.